Effect of γ'- and β-phase particles on the development under stress of FCC–BCT martensitic transformation in the [001]-oriented FeNiCoAlTa high-entropy alloy crystals

The results of a study on the influence of γ'- and β-phase particles on the development under stress of the FCC-BCT martensitic transformation (MT) and the temperature range of superelasticity (SE) in high-strength, high-entropy, nonequiatomic Fe-28Ni-17Co-11.5Al-2.5Ta (at.%) crystals oriented for tension along the [001] direction in the FCC phase are presented. It is shown that one-step ageing at 973 K for 10 h and two-step ageing at 973 K for 5 h and then, at 873 K for 3 h leads to the creation of a nanocomposite structure consisting of the FCC phase and particles of two phases: the γ'-phase (atomically ordered by the L 12 type) and the β-phase (atomically ordered by the B 2 type). After heat treatments, SE is observed in the temperature range from the M s temperature of the onset of forward transformation upon cooling to 298 K after ageing at 973 K for 10 h and to 393 K after two-stage ageing. In crystals aged at 973 K for 0.5 h, only γ'-phase particles are precipitated and SE is observed in the temperature range from 77 to 323 K. A thermodynamic analysis was carried out, which showed that the appearance of SE at T ≥ M s is associated with the influence of β-phase particles on the level of elastic energy at MT Δ G el.

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